Phthalocyanine compounds are useful as coatings, printing inks, catalysts or electronic materials and have recently been given extensive study particularly for their use as electrophotographic photoreceptor materials, optical recording materials and photoelectric conversion materials.
It is known that phthalocyanine compounds generally exhibit several different crystal forms depending on the process of production or the process of treatment and that the difference in crystal form has a great influence on their photoelectric conversion characteristics. For example, known crystal forms of copper phthalocyanine include .alpha., .pi., .chi., .rho., .gamma., and .delta.-forms as well as a stable .beta.-form. These crystal forms are known capable of interconversion by a mechanical strain, a sulfuric acid treatment, an organic solvent treatment, a heat treatment, and the like as described, e.g., in U.S. Pat. Nos. 2,770,629, 3,160,635, 3,708,292, and 3,357,989. Further, JP-A-50-38543 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") has a mention of the relationship between a crystal form of copper phthalocyanine and its electrophotographic characteristics.
Besides copper phthalocyanine, it has been proposed to use various crystal forms of metal-free phthalocyanine, hydroxygallium phthalocyanine, chloroaluminum phthalocyanine, chloroindium phthalocyanine, etc. in electrophotographic photoreceptors. In the course of study on the relationship between crystal forms of various phthalocyanine compounds and their electrophotographic characteristics, the present inventors previously discovered novel five crystal forms of hydroxygallium phthalocyanine and revealed their excellent characteristics as a photoreceptor as described in GB 2 255 980 A.
While JP-A-1-221459 refers to the relationship between the crystal form of hydroxygallium phthalocyanine and the electrophotographic characteristics, the mention is confined to those crystals obtained only by acid pasting. On the other hand, JP-A-59-168461 and JP-A-59-180566 disclose a process for obtaining a phthalocyanine pigment having a specific crystal form which comprises mixing a phthalocyanine derivative having an electron attracting substituent with an inorganic acid capable of forming a salt with phthalocyanine and precipitating the desired specific crystal by addition to water or a basic substance. However, there is mentioned no specific procedure for precipitating the desired pigment with the basic substance.
It has been difficult to obtain hydroxymetal phthalocyanine crystals having stable characteristics by ordinary acid pasting. That is, the resulting crystals, though having the same crystal form, show large scatter in characteristics, particularly charging properties and dark decay rate, when used as an electrophotographic photoreceptor. Moreover, any of the phthalocyanine compounds proposed to date, inclusive of the above-mentioned hydroxygallium phthalocyanine crystals, is still unsatisfactory in photosensitivity and durability as a photosensitive material.
It has thus been demanded to develop a phthalocyanine compound with stable charging characteristics and a small dark decay rate as well as improved photosensitivity and improved durability.
The present inventors conducted extensive studies for obtaining phthalocyanine compounds with stable characteristics and proposed a phthalocyanine compound obtained by wet grinding as disclosed in GB 2 255 980 A. However, the proposed process requires much time in wet grinding and tends to cause image defects due to incorporation of foreign matters, such as media, unless meticulous care should be devoted.
The inventors continued their study on a simpler and easier process for stabilization of characteristics and found as a result that a hydroxymetal phthalocyanine compound with highly stabilized characteristics can be obtained by a simple process comprising subjecting a precursor of a hydroxymetal phthalocyanine to acid pasting using water as a precipitating solvent and heat treating the precipitated crystal in a basic solution. They further found that replacement of water with a basic aqueous solution in the above-described acid pasting brings better results.
It is generally well known to obtain a pigment by subjecting a phthalocyanine compound, such as oxytitanium phthalocyanine or metal-free phthalocyanine, to acid pasting. In the case of these phthalocyanine compounds other than hydroxymetal phthalocyanine compounds, hydroxymetal phthalocyanine compounds with stable characteristics can be obtained simply by thoroughly washing the pigment after acid pasting followed by neutralization with, for example, diluted aqueous ammonia at room temperature.
According to the inventors' study, the above-mentioned difference between general phthalocyanine pigments and hydroxymetal phthalocyanine pigments was proved ascribable to the marked difference in the state after acid pasting. That is, general phthalocyanine pigments precipitated in an acidic solution after acid pasting assume a bright blue color, whereas hydroxymetal phthalocyanine pigments assume a dark green color in an acidic solution after acid pasting and turn to bright blue after being rendered weakly basic.
This indicates that the hydroxyl group possessed by hydroxymetal phthalocyanine pigments is much more susceptible to protonation as compared with general phthalocyanine pigments. It is assumed therefore that the acid resulting from the protonation is not sufficiently removed simply by washing with water after acid pasting followed by neutralization with diluted aqueous ammonia and that the characteristics of the pigment are subject to variation due to the trace residual acid.
The above-mentioned character of hydroxymetal phthalocyanine pigments may be applied to a pH sensor, a gas sensor, etc.